Mechanosynthesis of Nanophase Powders M F

Mechanosynthesis of Nanophase Powders M F. Miani F. Maurigh DIEGM University of Udine, Udine, Italy INTRODUCTION THE MECHANOSYNTHESIS PROCESS Among the different processes able to produce nanocrys- The concept of the mechanosynthesis process is very talline powders in bulk quantities, mechanosynthesis, i.e., simple: to seal in a vial some—usually powdered—ma- the synthesis of nanograined powders by means of terials along with grinding media, which are usually balls, mechanical activation, is one of the most interesting from and to reduce the size of the materials by mechanical an industrial point of view. Mechanosynthesis of nano- action, providing at the same time an intimate mixing phase powders is one of the less sophisticated technolo- of the different materials introduced, so close that gies—and, in such a sense, also the most inexpensive—to diffusion and chemical reactivity are greatly enhanced. produce nanophase powders; in fact, it exploits devices For clarity, one could consider a mixture of elemental and processes that have many aspects in common with powders—for simplicity, we could limit to powders of mixing, fine grinding, and comminution of materials. element A and element B—introduced in a cylindrical However, it does have interest for applications, as, in container, the jar of a vibrating mill, with repeated and principle, this is a very low cost process and its potential prolonged impacts imposed by mechanical energy im- for industrial applications is very significant. This article parted to grinding media (Fig. 1). describes the basis of the process, some possible applica- Depending on the nature of the reactants, different tions and perspectives of mechanosynthesized nanophase intermixing mechanisms are possible; generally speaking, powders, and some simple ideas that are able to depict part the ductile phase tends to incorporate—if such a phase is of the process, with a specific mind to the mechanosynth- present—a more brittle phase, with a tendency to form esis of nanocrystalline materials starting from elemen- particles which are microscopically (to a scale of microm- tal powder precursors. Mechanosynthesis belongs to the eters) folded. family of process technologies known as mechanical al- With prolonged milling, the crystal size of both re- loying, which are basically using the same or very similar actants decreases in an exponential way, following a de- apparatuses, i.e., milling or size reduction devices, which creasing trend with the processing time t of the type:[8,9] could be generally described as ball mills. A recent [1] ðÀc1tÞ thorough analysis by Suranarayana of the processes d ¼ df þðdi À df Þe ð1Þ related to mechanical alloying defines mechanosynthesis as a mechanochemical synthesis, attributing most of where d should be considered representative of the crystal the research work in this field of mechanical alloying size, being di the initial, df the steady-state crystal size, to Dodd and McCormick,[2] Takacs,[3] and Matteazzi and c1 an empirical constant depending on the process et al;[4] it is also necessary to quote the fundamental parameter. Other possible trends of the crystal size and pioneering work of Butyagin[5] and Urakaev and evolution with milling time are discussed in the next Boldyrev[6] and some important developments by Calka paragraph, especially if exothermic effects of the reaction and Wexler.[7] While, because of many different process A + B ! AxBy are to be taken into account. parameters and conditions, a complete modeling of the The gradual transformation of the milled powders from mechanical alloying process and mechanosynthesis of a rather coarse to a nanocrystalline grain size could be nanophase materials would be an aim very difficult to simply depicted as a formal reaction: reach, it is useful to identify the operating principle of A þ B ! A nano þ B nano the most common devices employed for the process, along with some kinematic aspects, some specific me- Miani and Fecht[10] have been considering mechanical chanical considerations, and thermodynamic and kinetic milling—i.e., the reduction of metal powders of elements issues of the process as well. Finally, current production of and intermetallic compounds to nanocrystalline powders mechanosynthesized powders and perspectives will be by grinding—and they have proposed empirical estimates briefly discussed. for the enthalpy stored. These estimates could be a base Dekker Encyclopedia of Nanoscience and Nanotechnology 1787 DOI: 10.1081/E-ENN 120009258 Copyright D 2004 by Marcel Dekker, Inc. All rights reserved. ORDER REPRINTS 1788 Mechanosynthesis of Nanophase Powders Fig. 1 Mechanosynthesis schematic process and steps. for an evaluation of the mechanochemical energy transfer efficiencies in the process that are decreasing from imparted to the powders. In mechanochemical processing, vibrating to tumbling—have been employed successfully. as time is increasing, along with the energy imparted to The different devices employed for research purposes are the materials, new phases/compounds tend to appear. It described in Koch;[11] there has been a tendency of has been generally recognized, by different authors, that different research groups[5,12,13] to develop ad hoc mills the generic reaction path linking the time evolution of the for the synthesis by milling of nanophase materials. Fig. 2 reactants with time is sigmoidal.[8–10] We shall come back describes a mill developed in the University of Udine, to this point dealing with the process kinetics. Here we which has been scaled-up to be industrially employed. could depict the second step of the reaction as: The most important physical parameters involved are well described in a series of papers by Cocco and nA nano þ B nano ! A B nano n Delogu,[8,9,14] in an attempt to propose a quantitative This is an oversimplification, as the mechanosynthesis understanding of the mechanical alloying processes. process occurs continuously. However, it may be useful to These authors have underlined the importance of relating evaluate, for instance, energetic aspects. basic and phenomenological aspects, suggesting that the framework of solid-state chemistry and chemical kinetics should be the most fruitful to interpret results and devel- KINEMATIC AND MECHANICAL ASPECTS op new trends. For the sake of simplicity, considering a mill with just one ball, and adopting the terminology of Milling devices are of different nature: vibrating mills, Butyagin,[5] one of the pioneers in the field of mechano- planetary mills, attritor mills, and tumbling mills—with chemistry, Table 1 was proposed. With such a situation, it ORDER REPRINTS Mechanosynthesis of Nanophase Powders 1789 could be interesting to adopt the quantitative techniques Table 1 Butyagin terminology for an elementary used in engineering the deformation processes, as it has mechanochemical process: ball milling with one ball been proposed earlier by Courtney and Maurice.[15] In any Physical quantity Expression Units M case, with the aid of simple kinetic trends, the milling 2 time should be related to the energy dose, not only for Impact energy E = 1/2mbvimp J device scale-up, but also for process control. This could Impact frequency N hits/sec 2 help, for instance, in controlling exothermic mechano- Milling intensity I = NE = 1/2Nmbvimp W 2 chemical reactions where combustion appears, imparting Energy dose D = It =NEt = 1/2Nmbvimpt J the completion of a mechanochemical synthesis of Specific dose Dm =NEt/mp J/kg nanocrystalline materials. mb and mp are the mass of the ball and of the powder, respectively. A possible new design for a jar for laboratory milling Source: Ref. [14]. is the one presented in Fig. 3, together with its con- structional details. The purpose of the experimental jar is to use a diatermic oil to monitor heat release during pro- arguments should not be of help. However, it is possible cessing. The jar has been produced with integral internal [16] to conceive the most simple reaction in the mechano- channels by means of selective laser sintering and it is synthesis of nanocrystalline materials—the one involving available for experimentation on SPEX 8000 mills; typical elemental powders A and B—as a two-step process: the dimensions of the main cylinder are 75mm in height and first could be thought of as a reduction of the crystal 60mm in outside diameter. domains and the second as the formation of a nanophase compound or alloy. Miani and Fecht’s[10] extrapolation may be employed for the first step, and Miedema mod- el for the second step, assuming that the enthalpy of alloy THERMODYNAMICS formation is the same also in the nanocrystalline status (Table 2). This approach is able to give order-of- Mechanosynthesis is a technique involving far from magnitude correct results, as could be checked compar- equilibrium processing; so, in principle, thermodynamics ing these semiempirical calculations[10] to the actual heat release.[17] Fecht proposed an empirical regression with an analysis by means of differential scanning calorimetry (DSC) data on nanocrystalline metals and intermetallic compounds obtained by 24-hr ball milling by SPEX milling. Scanning up to 870 K with 20 K/min, he obtained the results listed in Table 3. These data allow to estimate the excess enthalpy stored DH: À5 DH=DHf ¼ 8:5 Â 10 Tm ð2Þ where DHf is the enthalpy of melting and Tm is the melting temperature. KINETICS For what concerns kinetic evolution, the situation is very complex. We have different and contrasting phenomena. Dealing with the mechanosynthesis of nanophase materials, we are interested to obtain a crystal size of the order of tens of nanometers, which is usually considered an effective value for applications. The crystal size evolution Fig. 2 Experimental vibrating mill for the synthesis of nano- with time could be depicted with Eq. 1 reported above. phase materials in significant quantities. (From Ref. [12].) If thermal effects are present, i.e., if the enthalpy of re- ORDER REPRINTS 1790 Mechanosynthesis of Nanophase Powders Fig.

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